How popular is Kagan?

Kagan is an uncommonly occurring given name for males but a very prominent surname for both adults and children (#10152 out of 150436, Top 7%). (2000 U.S. Census)

Which version is better?

with its source form and related boy names. Prominent alternative forms of Kagan (outside Top 2000) are Keagan(#797 last year), Keagen, Keegan(#272) and Kegan. Other variants, like Keeghan, are seldom used. Usage of these forms of Kagan reached its peak in the year 2008 (adoption of 0.12%) and is somewhat less today (adoption 0.09%, ▼29%). Keegan is the most fashionable boy name here. (Top Baby Names, 2015)